1. Field of the Invention
The present invention relates to an ion source, and particularly relates to an ion source with a particular grid assembly, which is intended to be used in optical thin film deposition.
2. Description of Prior Art
Currently, optical thin film coating technology has been widely applied in high technology fields, such as laser optics and fiber communications fields. A thin film coating can be either a single-layer or a multi-layer coating on a substrate where several thin films are deposited on top of one another to achieve the required reflection/transmission via interference effects. The substrate may be a glass, metal, ceramic, or plastic substrate. An optical glass substrate is the most commonly used substrate. Each layer of the thin film coating is very thin, typically in the range between several nanometers and several microns. The number of layers may be one or a few hundreds. One of the applications of optical thin film coating is the optical filter, which is formed by depositing multiple layers of metal or medium materials on an optical substrate.
PVD (Physical Vapor Deposition) and CVD (Chemical Vapor Deposition) are two most common types of optical thin film coating method. PVD embraces evaporative deposition, sputtering and ion plating in reactive or inert environments. The evaporative deposition is generally carried out under a pressure of 10−3 to 10−4 Pa. The following sequential basic steps take place during evaporative deposition: a vapor is generated by boiling or subliming a source material; the vapor is transported from the source to the substrate; and the vapor is condensed to a solid film on the substrate surface. Sputtering deposition applies an inert gas at 10˜1 Pa in the vacuum chamber. The material to be sputtered is the cathode. The substrate to be coated is placed on the grounded anode. A glow discharge is formed by applying a high voltage between the anode and cathode. An electron source supports ionization of the inert gas, and the positively charged ions are accelerated towards the cathode. These ions bombard the cathode material with high energy and force atoms and molecules to break away from the cathode by virtue of their kinetic energy. Some of these sputtered particulates are intercepted by the substrate and form a uniform thin film layer. Ion plating is based on the evaporative deposition technology. In ion plating process, positive ions are produced in a glow discharge and are attracted to the substrate, which is connected as the cathode. The ions are typically made by evaporation.
As the quality requirements for optical thin film on optical components are becoming more and more stringent, an ion source (ion gun) is typically employed in the film coating process to improve the film quality. The film coating processes applying ion source include Ion Assisted Deposition (IAD), reactive ion plating, and so on. The IAD process has been especially effective in depositing dense coatings for optical coating applications. Bombardment can be done using a beam of condensable “film-ions” from an ion source (ion gun) disposed in a vacuum chamber and being independent from the source material. Ion bombardment of the substrate and the growing film adds thermal energy to the surface region without having to heat the bulk of the material. The ion bombardment also causes atomic rearrangement in the near-surface region during deposition, which increases the film density. This technology has the advantage of providing excellent control of deposition parameters. In addition, the coating of temperature-sensitive substrates, such as plastic, glass and semiconductor substrates, is possible with IAD processes because heating of the substrates is no longer necessary.
Broad beam ion sources used in IAD technology include Kaufman-type ion sources (referring to U.S. Pat. No. 4,446,403), cold hollow cathode type broad ion sources, RF ion sources, and gridless End-Hall ion sources. The type of ion source to which the target is connected depends heavily on the element to be ionized. Another important quantity is the beam purity that the ion-source selection most often determines.
FIG. 1 is a schematic block diagram of a conventional high-frequency ion source for explaining the operational functions of such a high-frequency ion source. FIG. 2 shows a cross-sectional representation of a discharge chamber of a conventional high-frequency (or radio-frequency) ion source 9 disclosed in U.S. Pat. No. 6,378,290, which is an improvement on that shown in FIG. 1. This conventional ion source 9 of the '290 patent includes a discharge chamber 90 at a closed end, a source 94 (shown in FIG. 1) for the gas to be ionized, a gas inlet 91 for supplying into the discharge chamber 90 the gas to be ionized, a high-frequency coil 92 surrounding the discharge chamber 90, a high-frequency generator 95 (shown in FIG. 1) connected to the high-frequency coil 92, for generating a high-frequency electromagnetic alternating field which ionizes the gas present in the discharge chamber 90, and an acceleration grid assembly 93 arranged at an open end of the discharge chamber 90 and connected to an acceleration voltage source 96 (shown in FIG. 1). The discharge chamber 90 is made of an electrically non-conductive material. The high-frequency coil 92 generates a high-frequency field, which ionizes a propellant present in the discharge chamber 90, preferably an inert gas. To ignite the discharge, free electrons supplied by an external electron source are accelerated through the high-frequency field, and collide with neutral propellant particles, i.e., inert gas atoms. When the discharge has ignited, it sustains itself, there is no need for any external supply of electrons. A plasma, comprising ions, electrons and neutral propellant, is finally generated in the discharge chamber 90. The fraction of ions in the plasma is determined by the output provided by the high-frequency field. Ions present near the acceleration grid assembly 93 are accelerated by the electrical field generated by the acceleration voltage, with a focused ion beam being formed. The acceleration grid assembly 93 comprises two to three thin plates each made of an electrically conductive material, with a plurality of holes provided therein. These holes are arranged so as to form extraction channels, which focus and accelerate the ions in the form of an ion beam emanating from the discharge chamber 90. Configurations of the plates forming a conventional acceleration grid assembly are also disclosed in U.S. Pat. No. 4,873,467.
A schematic diagram of a conventional radio-frequency (RF) ion source is shown in FIG. 3 to further illustrate the working status of such an RF ion source in detail. This RF ion source 8 comprises a discharge chamber 80 of an electrically non-conductive material, a gas inlet 81 communicating with the discharge chamber 80, an RF coil 82 surrounding the discharge chamber 80, a grid assembly provided at an open end of the discharge chamber 80, and an outer shell 83 surrounding the discharge chamber 80 and the RF coil 82. The outer shell 83 serves to provide shielding to the exterior of the electromagnetic fields generated in the discharge chamber 80, and to provide heat dissipation to the exterior of the loss heat arising during ionization. The grid assembly includes a screen grid 84 having apertures and disposed near the plasma in the discharge chamber 80, an accelerator grid 85 having apertures aligned with those of the screen grid 84, and a decelerator grid 86 beyond the accelerator grid 85. The screen grid 84 is kept at anode potential, the accelerator grid 85 is kept at cathode potential, and the decelerator grid 86 is equal to the ground. When the ions reach the screen grid 84, a strong electrical field generated between the screen grid 84 and the accelerator grid 85 extracts and accelerates the ions in the form of an ion beam. The decelerator grid 86 is adapted to better control the focusing of the ion beam, yield better uniformities and reduce erosion of the accelerator grid 85.
The above ion beam accelerated by the grid assembly will directly bombard the substrate and the growing film on the substrate, thereby adjusting the surface characteristics of the substrate. However, the ion beam tends to diverge or becomes larger due to the mutual repulsion of ions bearing the same charge. U.S. Pat. No. 6,319,326 discloses such an ion beam, which may be called a diverged ion beam. FIG. 4 shows another conventional ion source 7 used in the IAD process for filter manufacturing. Materials of high reflectivity and low reflectivity are sublimed and deposited on a glass substrate 70. The grid assembly 71 of the ion source 7 consists of three grids. Each grid is made of molybdenum and is curved at a central portion 72 thereof. The three grids of the grid assembly 71 are stacked with one another, and are spaced and insulated from one another by insulators positioned therebetween. The grid assembly 71 is disposed at an open end of the discharge chamber 73 in an outwardly curved profile, whereby the ion beam 74 diverges outwardly from the grid assembly 71 with a sequentially increased cross-sectional area. Although the surface area of the substrate 70 is entirely covered by the ion beam 74, this large cross-sectional area of the ion beam 74 disadvantageously causes energy loss and thus loss of adhesion of the resultant layer over the substrate 70 due to deposition density reduction. This results in reduced yield and throughput, which is unwanted.
Accordingly, an improved grid assembly for IAD application is desired to increase the yield and throughput of the resultant product.